Motor Vibration,high peak of 9.78 mm/s at exactly 100Hz 1

[Feg]

Hi Guys,

I have a new motor we put into service 1 week ago, we took out the original to replace bearings as it had been working for 5 years (routine service). This motor has shown up the following after routine condition monitoring:high peak of 9.78 mm/s at exactly 100Hz (twice line frequency) measured at the motor drive end in the horizontal plane(pump and motor laser aligned.) Do you have any ideas as to what maybe causing this (could it be a stator problem).

Thanks

 

[electricpete]

One of the first things to check would be soft foot. With proper safety precautions, carefully loosen one foot at a time and check for decrease in the twice line frequency. Retighten and go to the next foot. A significant drop in twice line frequency indicates the problem is probably created by uneven or angled foot conditions which distort the frame. By the way, is this a 2-pole (3000rpm)?

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[electricpete]

Much less likely candidates – unbalanced voltage, winding problem, as-built air-gap problem. One other easy thing to check that might indicate unbalanced voltage or winding problem would be a check of current balance.

(Again the number one suspect is foot conditions.)

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[waross]

Is this an induction motor?
If the frequency is twice motor speed, look for a mechanical or balance problem. If it is twice line frequency look for a stator problem or some problem in the magnetic circuit.
The slight difference between line frequency and motor speed is significant.
respectfully

 

[Feg]

Thanks electricpete,

Yes it is a 2pl in 200 frame, the point is, it is at exactly 100hz.

 

[waross]

I would then suggest a magnetic problem originating in the stator. This does not rule out electricpete's suggestion. A twisted frame may be distorting air gaps.
respectfully

 

[ScottyUK]

Is it at exactly 100Hz? Or just as close as you can measure? What did you measure it with? If the vibration is caused by the rotation of the motor it will be at (line freq. - slip freq.) which will be slightly lower than 100Hz, unless this is a synchronous machine in which case slip will be zero. Exact is a risky word to use in an engineering context - almost everything outside of the world of theory has a tolerance on it.

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[spciesla]

Pete is right on about the possible soft-condition. Another way to check for soft-foot if you're not running is to use a dial indicator mounted on each motor foot. Check to see if you get any vertical movement when you loosen the hold-down bolts.

 

[ozmosis]

feg

Is the motor you replaced identical in spec and manufacture as the one you removed?
Slim one this one, but worth a comment: If it is a different manf (meaning: you didn't have this problem before) then you can get a phenomena called "beat" at line frequency (50Hz) or direct multiples of this (100Hz, 150Hz), especially if the motor is of low quality (low copper content etc). It's difficult to overcome sometimes other than changing the motor make or perhaps adding an output reactor.